"Drinking energy drinks and alcohol together may provide a double-edged effect," the researchers write.

"The increased stimulation from energy drinks may negate some intoxication-related sedation side effects by increasing alertness. However, it could also lead to negative physiological side effects associated with over-stimulation."

The researchers surveyed 403 Australians aged 18 to 35 and compared their consumption of alcohol and energy drinks (AmEDs), with their consumption of alcohol only, in the preceding 6 months.

In Australia, an energy drink contains a similar amount of caffeine to a cup of instant coffee (about 80 milligrams of caffeine per 250 millilitres), says Amy Peacock, who worked on the research as part of her PhD in the School of Psychology. She says the recommended maximum daily intake is two energy drinks per day.

Peacock and colleagues found when people consumed AmEDs they drank, on average, 2.5 energy drinks with 7 standard drinks of alcohol in one sitting. Maximum consumption levels were 10 energy drinks and 20 standard drinks of alcohol.

They found when people drank AmEDs they were more likely to have symptoms of over-stimulation including heart palpitations, sleep difficulties, agitation, tremors, increased speech speed, irritability and tension.

She says these findings are in keeping with the overall results from previous international research.

The researchers also found AmED consumption countered the sedative effects of alcohol, with users having less speech and walking difficulties, confusion, exhaustion and sadness.

Risk-taking behaviour

The researchers also investigated the link between AmEDs and alcohol and risk-taking behaviours, including those involving driving, sexual behaviour, aggressive behaviour, illicit drug use and gambling.

Interestingly, they found that people drinking AmEDs were less likely to engage in these risky behaviours - despite drinking half a standard drink more than when drinking alcohol only.

These findings fly in the face of the main message coming from previous international findings, which suggest AmED consumption is associated with greater risk taking.

"It was a very surprising outcome for us," says Peacock.

Peacock and colleagues are currently carrying out laboratory studies to investigate further the effects of AmEDs and alcohol use on the ability to carry out computer tasks that assess risk taking.

Study limitations

Professor Steve Allsop of the National Drug Research Institute says he is not sure why the latest study has found less risky behaviour associated with AmED drinking.

But, he says, the most likely explanation is the selection bias that occurs in studies of this design, which rely on self-selected survey respondents.

"People interpret past behaviours differently," says Allsop.

But he says the findings do suggest there are no significant benefits to mixing energy drinks and alcohol, whereas there are significant risks, particularly for people drinking large amounts.

"The key message from all of the studies is caffeine is not an inoculation against the effects of alcohol," he says.

Wide awake drunk

Allsop says AmEDs may keep people awake longer so they drink more, and are likely to feel less drunk than they really are.

"They could then take the risks of driving or whatever in the misunderstanding of how intoxicated they actually are precisely because of the effects of the energy drinks," says Allsop.